The human spine is a biomechanical structure consisting of thirty-three vertebral members and is responsible for protecting the spinal cord, nerve roots and internal organs of the thorax and abdomen. The spine also provides structural support for the body while permitting flexibility of motion. In certain surgical procedures it is necessary to secure together two or more of the vertebral members. The procedure may be necessary for example as a result of physical trauma or degenerative diseases.
One type of surgical procedure includes attachment of a vertebral plate to the vertebral members. The vertebral plate is sized to extend across two or more of the vertebral members. One or more bone screws extend through apertures in the plate and into the vertebral members to secure the plate. One issue with the attachment is the screws may tend to loosen and back-out of the vertebral members. Screw retention devices may be necessary to prevent the screw from backing-out of the vertebral members.
One type of screw retention device utilizes a snap ring that expands as the screw is inserted into the aperture and then retracts to a smaller diameter once the screw head has passed the level of the snap ring. One issue with previous snap ring designs is the inability to use the rings with a variety of different screws. These designs may not be effective for both variable angle and fixed angle screws, or require specialty screws that are design for only a limited application. Additionally, these previous designs have needed a larger plate thickness to accommodate the snap ring. Another issue is the difficultly for the surgeon to visually see when the lock ring has moved over the top of the screw once the screw has passed the level of the snap ring. Some designs also prevent or limit the ability of the surgeon to tactilely feel the movement of the snap ring as it moves over the top of the screw. Additionally, some designs interfere with the surgeon's feel of the screw purchasing within the bone.